Process for production of ammonium thiosulphate

ABSTRACT

A process for continuous production of ammonium thiosulphate, (NH 4 ) 2 S 2 O 3  (ATS) from NH 3 , H 2 S and SO 2  comprising steps of:  
     (a) partial condensation in a partial condenser  4  of a first gaseous or partial liquid feed stream comprising H 2 O, H 2 S and NH 3  with a molar H 2 S:NH 3  ratio &lt;0.4;  
     (b) passing the aqueous condensate comprising NH 4 HS and NH 3  from the partial condenser  4  to a reactor  9  in which said condensate is contacted with a third feed gas stream  7  comprising H 2 S and with an aqueous solution  10  comprising NH 4 HSO 3  and (NH 4 ) 2 SO 3  under formation of an aqueous solution of (NH 4 )  2 S 2 O 3 ;  
     (c) passing the gas stream comprising NH 3  and H 2 S from the partial condenser  4  to a mixing device  13  in which said gas stream is completely dissolved in the water drained off from the aerosol filter  25;    
     (d) passing a second feed gas stream  20  comprising approximately ⅔ mole SO 2  per mole of NH 3  contained in the first feed stream to a SO 2  absorber  21  and the aerosol filter  25;    
     (e) passing the aqueous solution produced in mixing device  13  to the SO 2  absorber  21;    
     (f) passing the off gas from the absorber  21  to the aerosol filter  25  and  
     (g) adding to the aerosol filter  25  a balance amount of water required for obtaining approximately 40-65 wt % (NH 4 ) 2 S 2 O 3  in the aqueous of solution of (NH 4 ) 2 S 2 O 3  being withdrawn from the reactor  9.

[0001] This is a continuation-in-part of U.S. application Ser. No.09/938,519, filed Aug. 27, 2001, now abandoned.

INTRODUCTION

[0002] The present invention relates to a process for continuousproduction of a concentrated solution of ammonium thiosulfate (ATS) fromoff gases comprising H₂S and NH₃ such as refinery SWS (Sour WaterStripper)-gas, which contains NH₃ as well as H₂S and H₂S gas streams.

[0003] The process according to the invention is distinguished byutilizing only the NH₃ in the SWS gas for the production of high purityATS solution, thus producing 7.25 kg 60% ATS solution per kg of NH₃ inthe SWS gas treated in the process. Furthermore, when the processaccording to the invention uses the effluent gas of a Claus plant asSO₂-source for the process, the total sulphur-recovery of the Clausplant and the ATS plant taken together is increased to more than 99.95%with only 86-95% sulphur recovery being required in the Claus plant.

BACKGROUND AND OBJECTIVE FOR THE INVENTION

[0004] It is known to produce aqueous solutions of ATS by reacting asolution of ammonium sulphite with sulphur in liquid form or withsulphides or polysulphides in aqueous solution as described inKirk-Othmer Encyclopedia of Chemical Technology, 4^(th) edition, 1997,vol 24, page 62 and in U.S. Pat. Nos. 2,412,607; 3,524,724 and4,478,807.

[0005] It is furthermore known from U.S. Pat. No. 3,431,070 to produceATS in a continuous process from gaseous feed streams comprising H₂S,NH₃ and SO₂. By the process of this invention ATS and sulphur isproduced from a first feed gas stream comprising H₂S and NH₃ and asecond feed gas stream comprising SO₂ in three absorption steps. In afirst absorber, NH₃ and H₂S are separated in a H₂S off-gas stream and anNH₃-rich solution of ATS. The main part of the solution is passed to asecond absorber, in which it is contacted with the SO₂-rich feed gasstream under formation of an off-gas that is vented and a solution richin ATS and ammonium sulphites, which in a third absorber is contactedwith the H₂S-gas from the first absorber and, optionally, withadditional H₂S. After removal of sulphur being formed in the thirdabsorber, the major part of the ATS-solution formed in the thirdabsorber is recycled to the first absorber, while a minor part is mixedwith a fraction of the NH₃-rich solution of ATS formed in the firstabsorber forming the product solution of ATS.

[0006] There are three major disadvantages of this process: Elementarysulphur is formed in the third absorber and must be separated from thesolution, the off-gas vented from the third absorber has a highconcentration of H₂S and the process is complicated with threeintegrated absorption steps.

[0007] It is also known from EP 0 928 774 A1 to produce an aqueoussolution of ATS from gaseous feed streams comprising NH₃, H₂S andpossibly SO₂. By the process of this patent, a concentrated solution ofammonium hydrogen sulphite (AHS) is produced from NH₃ and SO₂ in a firstabsorption step comprising one or two absorbers in series. Said solutionis contacted in a second absorption step with a gaseous mixture of H₂Sand NH₃ forming the product solution of ATS.

[0008] The major disadvantage of this process is that it requires importof NH₃ for the process.

[0009] Furthermore, a process is known from Danish Patent No. 174407,wherein ATS is produced by using only the NH₃ contained in the SWS-gasstream as the NH₃ source for ATS-production.

[0010] In said process a first feed stream, typically SWS-gas,comprising more than 0.33 mole H₂S per mole of NH₃ is contacted with astream of sulphite solution in line 18 in the drawing FIG. 1 and FIG. 2in said patent application in a reactor A1 for formation of ATS.However, experiments have shown that the presence of excess H₂S for theformation of ATS from sulphites in the reactor will lead to the presenceof free sulphide in the product solution (line 12), part of which isrecycled (line 17) to the SO₂ absorber (A2), in which the sulphide willbe decomposed to give H₂S in the absorber effluent gas (line 19).Furthermore, the large recycle of solution (line 13) to the SO₂ absorberand back to the reactor (lines 17-18) is also a disadvantage of theprocess.

[0011] The objective of this invention is to establish an improvedprocess for the production of ATS in which over 99.9% of all sulphur andall NH₃ in the feed streams are recovered as ATS without any of theabove mentioned disadvantages.

SUMMARY OF THE INVENTION

[0012] This invention relates to a process for continuous production ofammonium thiosulphate, (NH₄)₂S₂O₃ (ATS) from NH₃, H₂S and SO₂ comprisingfollowing steps:

[0013] (a) partial condensation in a partial condenser 4 of a firstgaseous or partial liquid feed stream comprising H₂O, H₂S and NH₃ with amolar H₂S:NH₃ ratio <0.4, preferably in the range 0.1-0.25;

[0014] (b) passing the aqueous condensate comprising NH₄HS and NH₃ fromthe partial condenser 4 through line 5 to a reactor 9 in which saidcondensate is contacted with a third feed gas stream comprising H₂Ssupplied through line 7 and with an aqueous solution comprising NH₄HSO₃(AHS) and (NH₄)₂SO₃ (DAS) supplied through line 10 under formation of anaqueous solution of ATS being removed from the reactor through line 12;

[0015] (c) passing the gas stream comprising NH₃ and H₂S from thepartial condenser 4 through line 6 to a mixing device 13 in which saidgas stream is completely dissolved in the water that is drained off fromthe aerosol filter 25 and passed to 13 through the line 26;

[0016] (d) passing a second feed gas stream in line 20 comprising inprinciple ⅔ mole SO₂ per mole of NH₃ comprised in the first feed streamto a SO₂ absorber 21 and the aerosol filter 25;

[0017] (e) passing the aqueous solution produced in mixing device 13through line 14 to the SO₂ absorber 21;

[0018] (f) passing the off gas from absorber 21 through line 23 and 24to an aerosol filter 25 to which is added, through line 30, the balanceamount of water required for obtaining about 60 wt % (40-65 wt %) ATS inthe aqueous solution removed from the reactor 9 in line 12.

[0019] Step (e) can preferably be carried out by adding said aqueoussolution to the liquid recycle loop 27 of the SO₂-absorber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

[0020] Referring to the drawing FIG. 1, a first feed stream offractionated SWS-gas in line 1 comprising, for example, 6 kmole/h NH₃associated with 1.1 kmole/h H₂S and 2 kmole/h H₂O, is treated with asecond feed gas stream in line 20 comprising SO₂ associated with watervapor and inert components such as N₂, CO₂ and O₂ and a 3d feed gasstream comprising H₂S in line 7. Feed water required for the process isfed in line 30 to the aerosol filter 25. Some feed water may also beadded in line 2 to the first feed gas stream. More than 99.95% of theamounts of NH₃ and SO₂ in the feed streams are recovered in the productATS stream exiting the process in line 36. Thus, the off gas from theprocess in line 29 contains a negligible amount of SO₂ and essentiallyno NH₃ and H₂S.

[0021] The original SWS-gas available in refineries usually have a molarH₂S:NH₃ ratio of about 1:1, which is higher than directly acceptable forthe process. Therefore, the original SWS-gas must first be fractionatedby known methods in columns not shown on the figure to give a feed gasstream in line 1 with less than about 0.35 mole H₂S per mole of NH₃ inthe feed gas, preferably with a H₂S:NH₃ molar ratio in the range of0.1-0.25. The two other off streams (not shown in FIG. 1) from saidfractionation are H₂S, which can be used as make-up for stream 7, andpractically pure water. Experiments with the process have shown thatwith H₂S:NH₃>about 0.35 in stream 1, it will be very difficult orimpossible to avoid presence of free sulphide in the ATS product streamin line 36 and/or presence of H₂S in the process exit gas in line 29. AH₂S:NH₃ ratio of about 1.1:6=0.18 in feed stream 1 seems very suitablefor conducting the process according the invention.

[0022] The amounts of H₂S, SO₂ and H₂O comprised in the feed streams forproduction of a 60% ATS solution from 6 kmole NH₃ in the first feedstream are calculated from the mass balance of the over-all process:

[0023] Equation (1):

6 NH₃+4 SO₂+2 H₂S+17.5 H₂O

3 (NH₄)₂S₂O₃+16.5 H₂O,

[0024] corresponding to the following amounts in kg:

[0025] Equation (1a):

102.14 kg NH₃+256.24 kg SO₂+68.16 kg H₂S+315.32 kg H₂O.

[0026] give 741.86 kg 60% ATS solution.

[0027] The first feed stream comprising 6 kmole NH₃, 1.1 kmole H₂S and 2kmole H₂O in line 1 is cooled in the cooler 3 to a temperature between20° C. and 60° C., or well below its dew point upstream of the separator4. In the separator 4 the first feed stream is separated in an aqueoussolution exiting in line 5 comprising NH₄HS and some NH₃ dissolved inpractically all of the water in the feed stream 1, and in a gas phaseexiting in line 6 comprising most of the NH₃ (approximately 4.6 kmoleNH₃) and a small amount of H₂S (approximately 0.1 kmole H₂S). Water maybe added in line 2 upstream of 4, but in the example the amount of wateradded to the process at this point is chosen to be zero. The liquidstream 5 comprising in the example 1.0 kmole NH₄HS, 0.4 kmole NH₃ and 2kmole H₂O goes to the reactor 9 in which it is reacted with (0.9+x)kmole H₂S, the third feedstream and as defined below, introduced in line7 and with the liquid sulphite stream 10 comprising 0.15 kmole ATS, 3.3kmole AHS (NH₄HSO₃), 0.5 kmole DAS ((NH₄)₂SO₃) and 11.65 kmole H₂O. Inthe reactor 9 ATS will be formed by the principal reactions:

[0028] Equation (2):

4 NH₄HSO₃+2 NH₃+2H₂S

3 (NH₄)₂S₂O₃+3H₂O

[0029] Equation (3):

4 (NH₄)₂SO₃+2 H₂S

3 (NH₄)₂S₂O₃+2 NH₃+3H₂O

[0030] resulting in formation of 741.8 kg of 60% ATS solution leavingthe reactor through line 12.

[0031] An excess amount of x kmole H₂S may be added to the feed streamof 0.9 kmole H₂S, which constitutes the third feed gas stream, in line 7in order to increase the rates of reactions 2 and 3. The x kmole H₂S,which may constitute 0-10% of equivalent amount of H₂S required for theprocess, is vented from the reactor through the vent line 11.

[0032] Additional sulphite solution may be added through line 35 to theATS product solution having e.g. a composition of 60% ATS (3 kmol ATSand 16.5 kmol water), in order to complete the conversion to ATS ofpossible traces of sulphide or H₂S in the stream 12 and/or to add 0-2%DAS to the product ATS solution exiting the process in line 36. Forsimplicity, no sulphite is added through line 35 in the present example.pH of the solution in line 12 and in the reactor 9 will typically be inthe range 7.6-8.6.

[0033] The off gas from 4 (4.6 kmol NH₃ and 0.1 kmol H₂S) is passed tothe mixing device 13 in which it is completely dissolved in the aqueousstream 26 from the aerosol filter 25. Stream 26 will usually comprise atleast 0.1 kmole AHS which reacts with the H₂S and NH₃ in stream 6 underformation of ATS according to for instance equation 2. A minor fractionof the sulphite solution exiting the SO₂-absorption is added to stream26 through line 28 in order to ensure complete removal of all H₂S orsulphide in the aqueous stream 14 before it is added to the SO₂absorption loop 27. Addition of excess ammonium sulphites through 28will also keep pH in stream 14 below of about 9.2. If pH is higher than9.2 in stream 14 (due to the high concentration of NH₃ in said stream),the formation of ATS from the sulphide may be inhibited leading toliberation of H₂S in the SO₂-absorber 21 and to presence of H₂S in theabsorber off gas.

[0034] The flow of SO₂ (second feed gas stream) required for the processis in line 20 fed to the SO₂-absorber 21 in which the SO₂ is inprinciple absorbed by the NH₃ comprised in the NH₃-rich off gas in line6 from the partial condensation in 4 of the feed stream 1. According tothe over-all mass balance of the present example given in equation 1,4.0 kmole SO₂ is required for the process. As the SO₂ feed stream isproduced by upstream combustion of H₂S or other sulphurous components,the SO₂ in line 20 will be diluted with inert gas comprising N₂, CO₂ andO₂ and with water vapor. In FIG. 1 it is assumed that the 4 kmole SO₂ isdiluted with approximately 100 kmole inert gases and 6 kmole H₂Ocorresponding to a H₂O dew point of 35-36° C.

[0035] The SO₂ absorber 21 is typically a fixed bed absorber where theabsorption liquid is recycled in a loop 27 comprising a circulating pumpand a cooler which maintains the temperature of absorption preferably at35-40° C., whereby no net condensation or net evaporation of H₂O takesplace in the absorber and in the subsequent aerosol filter 25.

[0036] In the example in FIG. 1, 15.5 kmole/h water is supplied to theaerosol filter 25 for instance by spraying the water on the filtercandles. As there will be practically no NH₃, SO₂ or aerosols in theprocess off gas in line 29, the aqueous solution exiting the absorber inline 10 (after subtraction of the fraction taken out in line 28) can becalculated from the mass balances to be 617.2 kg/h solution comprising0.15 kmole/h ATS, 3.3 kmole/h AHS, 0.5 kmole/h DAS and 11.65 kmole/hH₂O. The equilibrium partial pressures of NH₃ of this solution at 40° C.have been found to be approximately 10⁻³ bars higher than that of SO₂.

[0037] In order to recover this NH₃, 0.1 kmole SO₂ is added to the SO₂absorber effluent gas by bypassing in line 22 about 0.1/4=2.5% of thegas flow in line 20 around the SO₂ absorber and adding said bypassstream to the absorber effluent gas in line 23. The NH₃ and SO₂ react inthe gas phase forming an aerosol of AHS, which is removed in the filter25. All aerosols present in the gas leaving the SO₂-absorber will alsobe removed and dissolved in the water supplied to the filter. The filteroff gas in line 29 contains typically about 40 ppm SO₂, less than 2 ppmNH₃ and essentially no H₂S.

[0038] Separation in the partial condenser 4 of the fractionatedSWS-feed gas stream in a gaseous NH₃-rich stream 6 and a liquid stream 5is very advantageous but not strictly necessary for the process.

[0039] The separation can in principle be replaced by splitting stream 1in a stream 5 being passed to 9 together with stream 7 and a stream 6being mixed in 13 in principle as seen in FIG. 1.

[0040] The SO₂-absorption may also be carried out with two SO₂-absorbersconnected in series, or the SO₂-absorber 21 may be a bubblingSO₂-absorber in which the feed gas in line 20 is bubbled through theabsorbing solution with or without external circulation in a loop 27 asshown in FIG. 1.

[0041] A fraction of the second feed gas stream can be by-passed theabsorber 21 and mixed with the effluent gas from the absorber 21upstream of the aerosol filter 25. The fraction can contain 0.7-1.3moles of SO₂ per mole of NH₃ contained in the off gas from theSO₂-absorber 21.

[0042] Essentially complete conversion of sulphide to (NH₄)₂S₂O₃ in theprocess and a desired concentration of excess (NH₄)₂SO₃ and NH₄HSO₃ of0-2 wt % in the product exit stream 36 are achieved (1) by adjusting thefeed rate of H₂S in line 7 to give a small stream in line 11 of excessH₂S in the range of 0-10% of the equivalent amount of H₂S for theproduction of ATS and (2) by bypassing a minor fraction of stream 10through line 35 to stream 12. In other words, the feed rate of H₂S inthe third feedstream 7 is adjusted to give an excess effluent H₂S streamfrom the reactor 9 of 0-10% of the equivalent amount of H₂S for theproduction of (NH₄)₂S₂O₃ and a fraction 10 of the solution produced inthe SO₂ absorber 21 is bypassed to the (NH₄)₂S₂O₃ solution withdrawnfrom the reactor 9.

[0043] Use of the present ATS process is in particular advantageous whenthe off gas from a Claus plant is used as source for the SO₂ requiredfor the process. This is seen in the overview in FIG. 2 of sulphurrecovery with a simple Claus process combined with the present ATSprocess in a refinery producing H₂S and SWS-gas from varioushydrogenation and cracking treatments of hydrocarbons. Without use ofthe ATS process all H₂S and SWS gas had to be treated in the Claus plantand the ammonia had to be decomposed at substantial costs. No more than95-97% sulphur recovery can be achieved in simple 2 or 3-bed Clausplants. Higher degrees of sulphur recovery require expensive processesfor tail gas treatment. Increasing the sulphur recovery to more than 99%by known processes is very expensive investment wise as well as withregard to operating costs and energy consumption. However, 99.95% totalsulphur recovery or more is automatically achieved with no increase inoperating costs and energy consumption by utilizing the off gas from asimple 2-bed Claus plant as the SO₂-source for the present ATS processutilizing the SWS-gas for ATS-production, as shown in the schematicdrawing in FIG. 2.

[0044] Referring to FIG. 2, the SWS gas containing e.g. 1 kmol ammoniaand 1 kmol hydrogen sulphide, in line 1 is sent to a fractionation unit2, in which it is split into an H₂S-rich stream 3 having e.g. ⅔ kmolhydrogen sulphide, and an NH₃-rich stream 4 containing all the NH₃ andapproximately 0.33 mole H₂S per mole NH₃. The H₂S-rich stream 3 is mixedwith an H₂S gas having e.g. 9 kmol hydrogen sulphide, in line 5 and sentto a Claus plant 6, in which most of the H₂S is recovered as sulphur inline 7. Air is also required in the Claus plant 6 and steam is produced.The sulphur recovery can be e.g. more than 93.1%. The off-gas from theClaus plant is sent to a tail gas incinerator 8, in which the H₂S iscombusted to SO₂ with an excess of air. Steam is produced. A fraction ofthe Claus feed gas is bypassed (hydrogen sulphide bypass) around theClaus plant in line 9 to the tail gas incinerator and combusted to SO₂.The bypass flow in line 9 is adjusted to give approximately ⅔ mole SO₂in the off-gas from the incinerator in line 10 per mole NH₃ contained inthe SWS gas in line 1. In order to produce concentrated ATS solution, afraction of the water contained in the off-gas from the incinerator isremoved in a condensation step 11. The off-gas from the condensationstep containing approximately ⅔ mole SO₂ is sent to the ATS process 12according to the present invention, in which the SO₂ is removed byreaction with the NH₃-rich stream 4 from the fractionation unit 2. Thepurified gas stream 13 is vented through the stack and the product ATSsolution (e.g. a 60% solution with 0.5 kmol ATS) is recovered in line14.

1. A process for continuous production of ammonium thiosulphate,(NH₄)₂S₂O₃ (ATS) from NH₃, H₂S and SO₂ comprising steps of: (a) partialcondensation in a partial condenser 4 of a first gaseous or partialliquid feed stream comprising H₂O, H₂S and NH₃ with a molar H₂S:NH₃ratio <0.4; (b) passing the aqueous condensate comprising NH₄HS and NH₃from the partial condenser 4 to a reactor 9 in which said condensate iscontacted with a third feed gas stream 7 comprising H₂S and with anaqueous solution 10 comprising NH₄HSO₃ and (NH₄)₂SO₃ under formation ofan aqueous solution of (NH₄) ₂S₂O₃; (c) passing the gas streamcomprising NH₃ and H₂S from the partial condenser 4 to a mixing device13 in which said gas stream is completely dissolved in the water drainedoff from the aerosol filter 25; (d) passing a second feed gas stream 20comprising approximately ⅔ mole SO₂ per mole of NH₃ contained in thefirst feed stream to a SO₂ absorber 21 and the aerosol filter 25; (e)passing the aqueous solution produced in mixing device 13 to the SO₂absorber 21; (f) passing the off gas from the absorber 21 to the aerosolfilter 25, and (g) adding to the aerosol filter 25 a balance amount ofwater required for obtaining approximately 40-65 wt % (NH₄)₂S₂O₃ in theaqueous of solution of (NH₄)₂S₂O₃ being withdrawn from the reactor
 9. 2.A process of claim 1, wherein the first feed stream of step (a) isdivided into a substream 6 being passed to the mixing device 13 and afurther substream 5 which is contacted in the reactor 9 with the thirdfeed stream 7 comprising H₂S and with an aqueous solution 10 comprisingNH₄HSO₃ and (NH₄)₂SO₃ under formation of an aqueous solution of(NH₄)₂S₂O₃.
 3. A process of claim 1, wherein a fraction of the secondfeed gas stream 20 is by-passed the absorber 21 and mixed with theeffluent gas from the absorber 21 upstream of the aerosol filter 25,said fraction containing 0.7-1.3 moles of SO₂ per mole of NH₃ containedin the off gas from the SO₂-absorber
 21. 4. A process of claim 1,wherein the SO₂-absorber 21 is a packed column.
 5. A process of claim 1,wherein the SO₂-absorber is a bubbling tank reactor with or withoutexternal liquid recycle
 6. A process of claim 1, wherein a fraction 28of the solution 10 comprising NH₄HSO₃ and (NH₄)₂SO₃ produced in theSO₂-absorber 21 is passed to the mixing device 13, said fraction of thesolution comprising a flow of sulphite (NH₄HSO₃ and (NH₄)₂SO₃) whichtogether with the sulphite in the off stream from the aerosol filter 25relates to the flow of sulphide (H₂S+NH₃HS) in the stream from thecondenser 4 by a molar ratio of 2:1 or more.
 7. A process of claim 1,wherein the second feed gas stream is an effluent gas stream from aClaus plant which is incinerated and its H₂O-content reduced to 3-10 vol% H₂O, preferably 6 vol % H₂O by cooling and partial condensation of itscontent of H₂O upstream of the present process.
 8. A process of claim 1,wherein the first feed stream 1 is Sour Water Stripper gas having beenfractionated and adjusted to contain H₂S and NH₃ to a molar ratio ofH₂S:NH₃ of <0.4.
 9. A process of claim 1, wherein pH value of theaqueous solution comprising NH₄HSO₃ and (NH₄)₂SO₃ used for absorption ofSO₂ in the absorber 21 is adjusted between approximately 5 andapproximately 7.5.
 10. A process of claim 1, wherein the feed rate ofH₂S in the third feedstream 7 is adjusted to give an excess effluent H₂Sstream from the reactor 9 of 0-10% of the equivalent amount of H₂S forthe production of (NH₄)₂S₂O₃ and a fraction 10 and 35 of the solutionproduced in the SO₂ absorber 21 is bypassed to the (NH₄)₂S₂O₃ solution12 withdrawn from the reactor 9.